You learned a hard lesson about yellow; you MUST use white sealer under it. Yellow is the poorest covering color out there and the only way to get reasonable coverage is with the whitest sealer you can get. Since I paint everything I can yellow, I am a yellow painting expert!
You can do just about anything you want to the clear-coat. sanding it and re-coating it w/ more clear is no problem at all. A tip for handling the runs; don't try to sand them flat. Won't work very well and you tend to cut through the surrounding clear before the drip is flat. However it is very easy to do a great job of eliminating the runs and drips. All you need to do is make a 'cabinet scraper' of a common single edge razor blade. To do this, take a brand new blade (get 100 of them from your paint supplier for a few $$) and draw the blade over a sheet of 600 paper in one direction several times. You should end up with a 'rolled over' edge on the blade. If you drag the rolled edge over a fingernail at a ~45* angle, it should have a slight drag resistance and scrape up a little nail dust. Bend the blade in a very slight, gentle arc with the rolled edge on the outside of the arc and your tool is ready. The bent arc prevents the ends of the blade from digging into the paint. Now simply drag the scraper over the run/drip and it will neatly totally eradicate the error. drag lightly, the sharp edge does all the work. Once all the blemishes are eliminated, color sand the surface smooth and re-clear.